There are many situations in which it is necessary to be able to determine and monitor the concentration of a substance in a fluid. One such situation is e.g. narcosis where five different anesthetic gases are administered in different combinations and mixtures to the respiratory circuit of a patient. In that case, it is of utmost importance that the concentration, i.e. the dose of the respective gas is kept under strict control.
FR 2 677 120 (Bussotti) discloses an arrangement for photometric spectroscopy, wherein measurements may be carried out on remote samples by using fibre optics. According to the Bussotti arrangement, the light from a lamp passes through an optical chopper that consists of a motor driven rotating disc with apertures which control light entry into the ends of two optical fibres. The two optical fibres illuminate a reference and a test sample, respectively. The output is recombined into a single optical fibre, for instance by the use of mirrors, for transmission to a measuring unit. Even though the arrangement described in FR 2 677 120 may be effective in many ways, the measuring principles are clearly different from the ones of the present invention, which will be described in greater detail herein below. With the Bussotti arrangement there is also a need for the illumination of a separate reference sample. According to the arrangement of the present invention, there is no need for such a reference sample but the reference measuring means are instead according to a preferred embodiment, in an effective way integrated within the optics of the arrangement.
The arrangement of the present invention can also be manufactured at low costs without having to sacrifice the high functionality and high measurement accuracy of the arrangement, especially with use of fibre optics.